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Heavy metal contamination is a threat to global food safety. Reducing heavy metal uptake in plants is a promising way to make plants safer, yet breeding the right set of traits can be tedious. We test whether microorganisms are able to impact the plant's hormonal balance hereby helping to manage plant heavy metal uptake. We focus on ethylene, a plant hormone regulating plant stress tolerance and nutrition. We grew three phylogenetically distinct plants, Rumex palustris, Alcea aucheri and Arabidopsis thaliana, on a cadmium-spiked soil. Plants roots were coated with the bacterium Pseudomonas putida UW4, which degrades the precursor of ethylene, or an isogenic ACC deaminase-deficient mutant lacking this ability. We followed ethylene concentrations, plant growth and cadmium uptake. Wildtype bacteria reduced shoot cadmium concentration by up to 35% compared to the control, while the mutant increased cadmium concentration. This effect was linked to ethylene, which was consistently positively correlated with cadmium concentration. We therefore propose that bacteria modulating plant hormonal balance may offer new possibilities to improve specific aspects of plant phenotype, in the present context reducing heavy metal. They may thus pave the way for new strategies to improve food safety in a context of the widespread soil contamination., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The plant hormone ethylene is one of the central regulators of plant development and stress resistance. Optimal ethylene signaling is essential for plant fitness and is under strong selection pressure. Plants upregulate ethylene production in response to stress, and this hormone triggers defense mechanisms. Due to the pleiotropic effects of ethylene, adjusting stress responses to maximize resistance, while minimizing costs, is a central determinant of plant fitness. Ethylene signaling is influenced by the plant-associated microbiome. We therefore argue that the regulation, physiology, and evolution of the ethylene signaling can best be viewed as the interactive result of plant genotype and associated microbiota. In this article, we summarize the current knowledge on ethylene signaling and recapitulate the multiple ways microorganisms interfere with it. We present ethylene signaling as a model system for holobiont-level evolution of plant phenotype: this cascade is tractable, extremely well studied from both a plant and a microbial perspective, and regulates fundamental components of plant life history. We finally discuss the potential impacts of ethylene modulation microorganisms on plant ecology and evolution. We assert that ethylene signaling cannot be fully appreciated without considering microbiota as integral regulatory actors, and we more generally suggest that plant ecophysiology and evolution can only be fully understood in the light of plant-microbiome interactions.